The Global System for Mobile Telecommunications (GSM) is currently the most widely used one of the operational digital cellular networks. Because of network congestion it has been imperative to change the operating frequency of the GSM system from the original 900 MHz, approx., to 1.8 GHz. Cellular networks complying with other standards are also widely used around the world. With the mobility of people and communication between people increasing, there is a growing need for general-purpose phones that operate in different networks according to network availability and/or service prices. In dual mode radio telecommunications, the GSM and DECT (Digital European Cordless Telephone), for example, or other systems with significantly different specifications, can operate as pairs. In dual band radio telecommunications, the systems are very much alike (e.g. GSM and PCN, Personal Communication Network), but the operating frequency of the higher-frequency system is a multiple of the lower-frequency system. The dual mode capability is also taken into account in the so-called third generation cellular systems (Universal Mobile Telecommunication System, UMTS/Future Public Land Mobile Telecommunications System, FLPMTS).
A dual mode radio communication device has to accommodate the duplexing and multiple access methods of the different systems. Duplexing means separation of traffic in the transmit direction from the traffic in the receive direction in the communication between two transceiver devices. Common methods include time division duplexing, TDD, and frequency division duplexing, FDD. Multiple access means sharing the capacity of a system or its part (a base station, for instance) between several terminals (such as mobile phones, for example). Commonly used methods include time division multiple access, TDMA, frequency division multiple access, FDMA, and code division multiple access, CDMA. In addition, the systems employ various multiplexing methods in which one device directs the transmitted information from several sources to a common transmission channel, separating the signals by means of, say, time division multiplexing, TDM, or frequency division multiplexing, FDM.
A prior art radio apparatus using full time division or frequency division duplexing includes several RF and IF filters both on the transmitter side and on the receiver side. FIG. 1 shows a prior art GSM radio. In the GSM system, transmission and reception are carried out in different time slots and at different frequencies. The radio apparatus 100 includes on the receiver side a band-pass filter 12 the input port of which is connected to an antenna switch 14. The output port of the filter is connected to a low-noise amplifier (LNA) 17 which amplifies the received radio signal. It is followed by a second band-pass filter 18 which further filters the received signal. The output port of the filter 18 is connected to a mixer 11 in which the received signal is mixed with a first injection signal coming from a synthesizer 22. The mixing result, which is an intermediate-frequency signal IF, is taken via a filter 24 to a RF circuit in the receiver for further processing.
The transmitter part of the radio 100 includes a second local oscillator signal (LO) 26 which is produced by the transmitter pre-stage (not shown) and mixed in the mixer 30 with the first injection signal. The output of the mixer 30 is taken to a band-pass filter 13 which is normally found prior to the transmitter power amplifier 16. The output of the power amplifier 16 is connected to the input of a low-pass or band-pass filter 15 so as to further filter out undesired components in the signal before transmitting it via an antenna 21. In between the power amplifier 16 and the low-pass filter 15 there is often a directional coupler (not shown) which can be used for measuring the power level of the signal brought to the antenna.
FIG. 2 shows a DECT radio according to the prior art. A radio apparatus 200 includes a band-pass filter 19 the input port of which is connected to an antenna switch 14. The output port of the filter is connected to an antenna 21. One output port of the antenna switch is connected to a low-noise amplifier (LNA) 17 which amplifies the received radio signal. It is followed by a second band-pass filter 18 which further filters the received signal. The output port of the filter 18 is connected to a mixer 11 in which the received signal is mixed with a first injection signal coming from a synthesizer 22. The mixing result, which is an intermediate frequency signal IF, is taken to a RF circuit in the receiver for further processing.
The transmitter part of the radio 200 includes a mixer 30 in which the I/Q-modulated transmission signal is mixed with an injection signal. The output of the mixer 30 is taken to a band-pass filter 13 which is normally found prior to the transmitter power amplifier 16. The output of the power amplifier 16 is connected to a second output port of the antenna switch 14.
The antenna switch, which connects the antenna alternately to the transmitter and receiver branches, is used in a mobile phone to separate the signals if the transmission and reception frequencies are the same. If the transmission frequency band is different from the reception frequency band, the separating unit may be a filter similar to the duplex filter used in analog phones. The latter option can also be used in systems employing frequency division multiple access. FIG. 3 shows a prior art GSM radio 301 which differs from the radio 100 shown in FIG. 1 in that in this apparatus 301 the antenna switch (14), band-pass filter (12) and low-pass filter (15) are replaced by a duplex filter 20. The rest of the functions of these two radios are identical. A duplex filter is a three-port circuit element in which there is a receive branch filter between the antenna port and the receiver port, and a transmit branch filter between the transmitter port and the antenna port. The operating frequencies of the filters are such that a transmission-frequency signal cannot enter the receiver port and a reception-frequency signal cannot enter the transmission port. The frequency characteristics of the filters may be adjustable.
FIG. 4 shows a prior art dual mode GSM/DECT TDD radio 400 wherein both systems use a common antenna. In the radio according to FIG. 4 the antenna filtering arrangements in both systems are based on antenna switches and separate filters. An antenna switch 41 connects the common antenna either to the GSM or to the DECT system. When choosing the DECT system, the rest of the functions of the radio are, mainly the same as those shown in FIG. 2 and comprise a band-pass filter 19, a second antenna switch 14b, a receiver chain 17b-18b- 11b-24b and a transmitter chain 13b-16b. When the GSM system is used the rest of the functions of the radio are mainly the same as those shown in FIG. 1 and comprise a receiver chain 12a-17a -18a-11a-24a and a transmitter chain 13a-16a-15a as well as a third antenna switch 14a which corresponds to the antenna switch 14 shown in FIG. 1. A switch 42 on the receive side and a switch 43 on the transmit side operate synchronously with the antenna switch 41, connecting the radio-frequency parts of either the DECT or the GSM system shown in FIG. 4 to the common modulation and demodulation parts of the dual mode phone and thence to other parts of the radio apparatus.
Even if a digital mobile phone using frequency duplex had an antenna switch to separate transmission and reception, it also must have filters since there has to be selectivity in the receiver input and it has to protect a low-noise preamplifier. Harmonic multiples of the output frequency and other spurious signals such as mirror frequencies have to be attenuated at the transmitter output. In addition, the filters eliminate noise generated on the receiver band by the transmitter chain. Also the frequencies below the transmission band have to be attenuated by a separate filter. In systems employing time duplex, such as DECT, or Digital European Cordless Telephone, it has to be made sure, in addition to the above, that spurious signals generated in the direction of the antenna by the receiver side during the transmission of the signal are sufficiently attenuated.
The standard impedance at interfaces between discrete components and filters is 50 ohms. Filter and semiconductor manufacturers match the input and output impedances of their products to the standard value in order to make modular design easier. In dual mode radio communications, the matching of a GSM duplex filter or transmission and reception filters, and, on the other hand, the matching of a DECT band-pass filter to a common antenna proves problematic. In prior art arrangements, impedance matching requires bulky and lossy separate components.
Thus, the prior art dual mode phone shown in FIG. 4 has to have as much as three separate antenna filters (reference designators 12, 19 and 15) and the matching circuits required by them. In addition, the construction includes all in all five radio-frequency switches. It is obvious that this kind of arrangement takes a lot of space on the printed circuit board of the radio apparatus and is expensive to manufacture. Furthermore, a high number of separate components increases losses and susceptibility of the circuit to electrical interference and to electrical or mechanical failure.